The present invention relates to signal processing techniques for providing high fidelity signal amplification. More specifically, the present invention provides techniques by which mixed signal amplification is employed with noise-shaping to generate an output signal with very low distortion.
Both switching and analog amplifiers have applications for which they are considered preferable. For example, because of power dissipation advantages, switching amplifiers are often employed for applications in which the fidelity of the output signal is not the greatest concern. That is, switching amplifiers generally exhibit lower power dissipation when outputting power of an appreciable level, but do not typically match the fidelity of analog amplifiers. Exceptions to this general rule are switching amplifiers provided by Tripath Technology Inc. of Santa Clara, Calif. Signal degradation due to increased harmonic distortion becomes especially pronounced for both switching and analog amplifiers as the output signal swing approaches the power supply rails, although some analog techniques allow rail-to-rail operation. The graph of FIG. 1 illustrates the effect of output signal swing on the total harmonic distortion of a typical switching amplifier.
The advantages of lower power dissipation are well known and include such things as, for example, smaller heat sinks and power supplies, reduced battery drain and operating temperature, and smaller product size. These significant advantages have led to the widespread use of switching amplifiers in a variety of applications. However, despite the design of some analog amplifiers, there are circumstances in which an analog amplifier may be designed with significantly less power dissipation than an equivalent switching amplifier, e.g., a class AB amplifier with a small bias. This typically occurs at or near quiescence, i.e., when there is little or no input signal but the amplifier remains active. This is due to the fact that, at quiescence, a switching amplifier must still produce a large switching voltage signal while an analog amplifier can "rest." Thus, for applications in which there is a considerable amount of idle or low power time, the use of analog amplifiers may be preferable. Of course, if the output swing for such an application at any time exceeds a certain level, and thus the power dissipation of the analog amplifier exceeds that of a comparable switching amplifier, the size of the heat sink and power supply must still be such that they could support such a power level on a consistent basis and these advantages are not realized. Furthermore, when a low bias is used, distortion problems can be exacerbated.
One approach to solving this dilemma will now be discussed with reference to the block diagram of FIG. 2. According to this technique, analog amplifier 202 is employed when there is little or no input signal to take advantage of its low quiescent current. When the output signal swing reaches a certain level, switching amplifier 204 is employed to take advantage of its lower power dissipation for higher output power levels.
Unfortunately, the approach of FIG. 2 is not feasible for high fidelity applications in which only very low levels of distortion are acceptable. This is because of the distortion in the output signal introduced by the transition between the analog and switching amplifiers. Moreover, such an approach does not address the fact that the fidelity of switching amplifiers drops off dramatically as the output signal swing approaches the power supply rails. In addition, with such an approach, the distortion of the analog amplifier goes uncorrected.
It is therefore desirable to provide a signal processing technique which exhibits the advantages of both switching and low power, high-signal-swing analog amplifiers while maintaining low distortion levels for high fidelity applications.